Educators especially: I encourage you to use anything you find here that is useful to your mission educating people about Earth science. E-mail me if it would help to have a larger or higher-resolution version of any of the images.

Picking up where we left off last time, we were in some partly-serpentenized peridotite, part of the Burham Ophiolite in Turkey’s Tavşanlı Zone, an ancient tectonic suture.

Our next stop on the field trip allowed us to visit some diabase dikes:

Here’s a close-up of the right contact of the dike with the host peridotite:

The field notebook’s long edge is ~18 cm. And here it is again, annotated:

Near the village of Oranheli, we stopped to examine a jadeite meta-granitoid, a rock only a metamorphic petrologist could love. There were, however, a lot of metamorphic petrologists on the trip, and they were very keen on checking it out. This was the first of many occasions when random Turkish citizens would stroll up to our odd group to find out just what the hell we were doing:

Further along, we saw a meta-basite (meta-basalt) within the meta-granitoid, and there I got a refreshing whiff of structure. Here’s a random isoclinal fold of a meta-granitoid dike cross-cutting the meta-basite, with a Turkish 1-lira coin (about the same size as a U.S. quarter) for scale:

Next up were some very cool rocks: marbles with extremely elongated calcite crystals.

These needle-like crystals are interpreted as being pseudomorphs of aragonite, the form of CaCO3 which is stable at high pressures and low temperatures.

A bit further on, we return to metamorphosed shale and graywacke (now schist and “grayfels”), sheared out and pervasively deformed at blueschist conditions. I took a few photos of charismatic folds in the unit:

Annotated, roughly showing the trace of foliation:

Sandy layer folded over into a recumbent position, set in a sheared mass of meta-shale:

Coming down onto this roadside outcrop of sheared shale and graywacke were cobbles and boulders of float from somewhere up above. They were of a quartz-pebble conglomerate that showed a stretching lineation. Check out these two faces of typical samples:

Now, here they are again, with the X, Y, and Z axes of the strain ellipsoid (longest, intermediate, and shortest, respectively) labeled for your benefit.

This conglomerate has been sheared into a lovely L-S tectonite, with X>Y~Z. In other words, it’s mostly lineated, with only a weakly-defined foliation, indicating the stress field was mostly constrictional. (I collected a muddy sample of this stretched-pebble meta-conglomerate, and when I washed it off in the hotel shower the next morning, I was delighted what a cool sample I had selected. It has some awesome structural features; I’ll show it to you some other time…)

Our final stop of Day 1 of the trip was this spectacular overview of the Kocasu Gorge, a canyon which cuts across the structural trend of the area at approximately a right angle. (The canyon cuts north-south; the strike of the folded & thrusted rock units runs approximately east-west.)

As the sun set, Aral showed us where we were, and the overall synclinal structure of the area.

I recorded it in my field notebook like this:

With this context established, we loaded back on the bus and drove for a couple of hours to get to a town with a decent hotel. We dined and slept, and the next morning got up ready for more suture-zone rocks.

While I was away, apparently the geoblogosphere went on a rampage of cooling columns. Everyone was posting images of their favorite columnar joints, and I was left out in the cold. Let me remedy that now. As it turns out, I was visiting some columns while everyone else was writing about them. Here are some images from the Güvem area of Turkey, north of Ankara, where there are a mix of late Miocene lake sediments and intercalated volcanic rocks, including these basalt flows. We stopped to visit them last Wednesday on our way to the North Anatolian Fault:

The dark entablature looms above:

A nice central panel with a good cross-section of the flow:

Around the corner, some more:

I ran across the street (and a stream) to check out a similar exposure there:

As mentioned, I spent a significant part of last weekend was spent on a paleomagnetic sampling project with collaborators from the University of Michigan. On Friday, this was our field area:

That’s the south slopes of Old Rag Mountain, a popular Blue Ridge hiking destination because unlike many Virginia peaks, when you get to the top, you see some rocks instead of 100% trees:

But we didn’t come here for the view. We came here for the dikes. Here’s the edge of one, with a pen for scale:

These are dikes of basalt and meta-basalt of the Catoctin Formation which are presumed to be feeder dikes pumping mafic lava to the surface of Virginia around 570 million years ago, during the breakup of the supercontinent Rodinia and the opening of the Iapetan Ocean basin. The dikes cut across the Grenville-aged basement rocks, in this case the Old Rag Granite of about 1000 million years age. The Old Rag area is especially great because the dikes are less metamorphosed than they are in other parts of the Blue Ridge province, where the Catoctin has been cooked into greenstone. Here’s an annotated view of the previous photograph:

As far as this project goes, we are interested in these dikes for the information that they (potentially) contain about the orientation of the Earth’s magnetic field in Virginia at the time of the supercontinent Rodinia’s breakup. By sampling these dikes and then analyzing the samples at their paleomagnetism lab back in Ann Arbor, Fatim and Matt hope to put some constraints on the question of paleo-Virginia’s latitude when these dikes cooled into solid rock.

As a reminder, you are not allowed to sample any rocks in any national park unless you have first applied for and been granted a research sampling permit by the National Park Service.

Close to the planet’s surface, the Earth’s magnetic field is shaped like a torus (or, in less technical terms, a doughnut, but one of those donuts with a pinched up midsection, and more of a dimple than a hole). It exits at the south magnetic pole, wraps north around the Earth, and plunges back into the inner core at the north magnetic pole:

A magnetically-sensitive mineral forming in a modern rock would have an upward-oriented high-angle magnetism if it formed at high southerly latitudes, a moderate-angled upward orientation at moderate southerly latitudes, a horizontal, northward-pointing orientation at the magnetic equator, and then the reverse as you head towards the north pole: a moderate-angled downward orientation at moderate northerly latitudes, and a downward-oriented high-angle orientation if it formed at high northerly latitudes, just like the red arrows show in the above image.

Of course, the flow of the magnetic field occasionally reverses direction (emerging at the north magnetic pole instead, and flowing south), but the shape of the field doesn’t change:

So the angle of inclination of a fossil magnet should be the same regardless of whether it’s poking up or plunging down, relative to the surface of the Earth. In this way, paleomagnetism can reveal the approximate latitude (but not longitude) at which a rock formed.

But wait, is it really so simple? No, of course not. Check out the map below, showing the positions of the north geomagnetic pole over the past 2000 years, with numbers showing the position of the pole in a specific year CE. It moves! The circles around geomagnetic poles at 900, 1300, and 1700 CE are 95% confidence limits on those geomagnetic poles; the mean geomagnetic pole position over the past 2000 yr is shown by the square with stippled region of 95% confidence. These data were compiled by Merrill and McElhinny (1983) and plotted by Butler (1982).

So this map shows us that even though the magnetic pole does wander about a bit, 2000 years of data is enough to generate an average which is more or less coincident with the geographic pole. And therefore a statistically significant batch of data (spread over a 2000-year-or-greater spread of time) will also reflect that average pole position.

Meert, Van der Voo, and Payne (1994) made a first attempt at constraining the paleomagnetics of the Catoctin Formation. Four of their 32 sites were feeder dikes, sills, and host rock (Grenvillian basement complex). One of the things these authors did was that they performed a “contact test” on two of their dikes. A contact test is a way of using an igneous contact (as with a dike) to determine whether the whole region has been magnetically reset, perhaps by thermal activity accompanying contact metamorphism. Consider this situation:

You sample a dike and its surrounding host rock, at several distances away from the dike. You find that they all give you the same magnetic orientation. This suggests you have the magnetic signature of a later overprinting, not the original orientations of dike and host rock.

Now what if you found this, instead?

Here, your dike shows a distinct signature that is different from the host rock, and the host rock shows a uniform orientation except right next to the dike, where the heat of the intrusion has partially reset the (older) host rock’s magnetism. If I were to annotate this up (with color coding!), it would look something like this:

Passing the contact test is critical to tying the two rocks’ magnetic data to their age data. It’s only with a positive contact test that you can use this data to say anything about where Virginia (and thus ancestral North America, often dubbed “Laurentia”) was when the Catoctin dikes were intruded.

The contact test is something that our team wanted to repeat, with more dikes than just the two that were featured in the Meert, et al. (1994) paper. We also wanted to double-check their results, and verify, reject, or modify them as our data warranted.

The key to constraining the magnetic orientation of these rocks as precisely as possible is to constrain the current orientation of the samples as precisely as possible. We measured the strike and dip of the surface of each sample very carefully, before we extracted it from the bedrock. At Old Rag Mountain, we were not allowed to drill (Old Rag is a wilderness area with no motorized equipment allowed), so we were collecting oriented hand samples.

Here’s Fatim Hankard writing orientation data in her field notebook while Matt Domeier takes a strike and dip reading in the background, using his Brunton compass:

Because these rocks are inherently magnetic (that’s why we’re sampling them, after all!), we have to control for the possibility that the rocks themselves might be throwing off our Brunton compass needles. A second compass is employed to control for any magnetic field coming off the rocks themselves. This is a solar compass. If you know exactly where you are (note Fatim’s GPS unit in the above photo), and when you are taking the measurement, you can use this solar compass to double-check the orientation you get from the Brunton compass.

Here’s Matt’s solar compass butted up against one of our Old Rag samples. Note the shadow being cast by the compass’s nomen, and also note the “arrow with a prong” strike and dip symbol that we wrote directly onto the face of the sample with a Sharpie:

Next, take a look at a photo of a sample once extracted. We label it redundantly, not only in terms of the orientation lines, but also in terms of the sample’s identity. That way, we’re less like to find a bunch of scratched-up but un-identified and un-orientable rock samples once the van gets back to Michigan:

While poking around, I found this interesting feature at the edge of one of the dikes. I’m hoping one of my more petrologically-inclined readers may be able to offer me some kind of interpretation of this pattern:

What I noticed is that in the first few mm of the dike, right up against the contact with the host rock, there are no white lathes of plagioclase feldspar. These relatively large feldspar crystals are phenocrysts, big chunky crystals that grow in the magma when it’s cooling relatively slowly underground, but then entrained in the flow as it moves upwards into the dikes, whereupon the surrounding liquid chills rapidly to make fine-grained basalt. So there are no phenocrysts right at the edge of the dike, then there are a bunch, all aligned with one another (but with no preferred sense of imbrication, so far as I can tell), and then there are more phenocrysts in the bulk of the dike, but they are (a) less concentrated, and (b) lack any preferred orientation. Let me annotate it for you, then go back and take another look at the unannotated version, so you can see what I’m referring to:

Okay, petrologists, I want to hear from you: How should I interpret this?

Back to the paleomag… On Saturday, we went to another location to sample. This one was much more convenient because (a) it was right on the side of the road, and (b) it wasn’t a wilderness area, so drilling was allowed. This was at the lovely selection of Catoctin dikes downhill (north) from the Little Devils Staircase overlook, on Skyline Drive in Shenandoah National Park. Here’s a charismatic dike with Matt acting as a sense of scale:

Annotated:

We unpacked the gasoline-powered diamond-grit-tipped drills and hooked them up to the water pump. We put on ear- and eye-protection, and got to work:

One the sample has been drilled out, you’re left with an empty hole. The white liquid is the cooling water with suspended dust from the abraded rock. This hole is about 3 cm in diameter:

The core (2.5 cm diameter) that came out of that hole:

In our field area, a core this size of the dike rock takes about ten minutes to extract. Basement rock (host rock) takes longer, as it’s made of harder minerals.

One worry is that the core will snap loose while you are drilling it out. If this happens, it may start rotating in the hole, and you will lose all sense of how it was originally oriented, which means you’ve just wasted a lot of time for no gain in data. To protect against this possibility, we used a technique of scoring a second circle with the drill bit, partially overlapping our actual core like a Venn diagram:

This way, if the core snaps off, you can line up its arc with the rest of the circle inscribed on the outcrop next to the hole. Whew! Core saved!

Fatim extracting another core:

After the core is drilled out (but still in its hole), Fatim oriented it. Notice the new array here – it’s a stand with slots into the drill-hole, then has a Brunton compass atop it with a solar compass atop that:

As you can see with this example, the solar compass is just about to become useless as the afternoon shadows advance! Next up, record all the orientation information (trend and plunge of the cylinder’s axis), and then score the core with a line:

Fatim and Matt sampled for two more days after I had to leave them due to other obligations, like teaching. They are headed back to Michigan today. Soon, hopefully, we’ll see whether our sampling campaign yields any meaningful results… Stay tuned!

As a final note, I would like to point out that this collaboration was born when Fatim read my blog post on feeder dikes and then proposed that we combine her paleomag skillz with my dike-location knowledge. It’s not the first time that my blogging has yielded a great opportunity, but it seems to be a shining example of how virtual connections online can lead to tangible work in the real world. The blog-curious should take note.

The past couple of days, I’ve been in the field, collecting samples with Dr. Fatim Hankard, a post-doctoral researcher from the University of Michigan, and Matt Domeier, a PhD candidate from that same fine school. We’re interested in using Virginia’s wealth of Catoctin formation feeder dikes to do paleomagnetism measurements that might help us constrain the latitude of Virginia during the emplacement of these dikes during the Neoproterozoic.

More later on the drilling technique and goals, but here’s a small batch of funny photos from Robin R., one of three Honors students who joined the researchers yesterday for drilling of Catoctin dikes along Skyline Drive in Shenandoah National Park*. The other two students were Elysia H. and Aaron Barth, former NOVA Honors student and now a George Mason University geology major. Thanks for the photos, Robin!

So here I am as a bad-ass driller. The reason I was feeling so aggressive was I was drilling out a beautiful core, when suddenly the rock face I was drilling in detached and the chunk of rock stuck to the drill, spinning around in the air. We all had a good laugh at that. It’s testament to what a nice core this would have been that you can see water burbling through the sample and dribbling down into the air behind it. Here, I’ll outline the sample (hard to see the dark rock against the dark background) and the water for you:

Another funny moment occurred when we fired up the drill while the bit was still lying in the tall grass. Instantly, it would up a nice mantle of grass into a tube, like a fork twirled in spaghetti:

Lastly, I’d like to demonstrate how far I have advanced in my own arachnophobia by showing how close I got my finger to this fat orb weaver spider that was crawling over the basement complex adjacent to one of the dikes:

…Okay, I’ll admit it: at one point, the spider changed direction, and brushed up against my finger, and I shrieked like a little girl. This prompted another round of laughs at my expense.

Great times, hopefully to yield great data… Stay tuned.

________________________________________

* Yes, we had a permit to collect in the park. It is illegal to remove rocks or other natural resources from national parks without explicit written permission from the National Park Service.

This quarry chews into rock right along the contact between a mafic igneous intrusion and lake sediments that formed when water pooled in a low-lying continental basin that formed during the breakup of Pangea. This rift valley, the Culpeper Basin, is just one prominent basin in a whole series of Triassic grabens and half-grabens that run through the Piedmont north and south of here, including all the way to the Bay of Fundy.

A similar environment can be seen today in east Africa, where a modern rift valley hosts similar lake deposits and mafic igneous rocks:

If you were to drop maybe half a kilometer below the surface of the Afar region, you’d see a similar situation to the one that produced Greg’s quarry photo ~200 million years ago.

Visiting the Centreville quarry is by permission of the Luck Stone corporation only; the best way to see it is by signing up for Ken’s course the next time it rolls around!

In the 1987 comedy Planes, Trains, and Automobiles, John Candy and Steve Martin have a funny experience. It involves a cozy hotel room (one bed only) and the two travelers are huddled up for warmth. As he wakes up, John Candy thinks he is warming his hand “between two pillows.” At hearing this, Steve Martin’s eyes pop wide open, and he yells, “Those aren’t pillows!”

They jump up, totally discombobulated. An awkward moment follows.

Well, it’s not quite as awkward, but I had a similar “those aren’t pillows” moment recently. I was out in Shenandoah National Park with my GMU structural geology students, and we stopped off at the Little Stony Man parking area (milepost 39.1 on Skyline Drive). Here’s a figure showing the area in question, from Lukert & Mitra (1986):

You’ll note in the detail map at the right that it shows the nonconformable contact that separates the basement complex (here, the “Pedlar” Formation) from the overlying metabasalts of the Catoctin Formation.You’ll also note that it says “PILLOWS” with an arrow pointing at a specific spot on the trail. The word refers to basaltic pillows, which are breadloaf-shaped primary volcanic structures that form when lava erupts underwater. They are typically the size of a bedroom pillow (especially overstuffed pillows). Here’s some video of pillows erupting.

Pillows have been reported elsewhere in the Catoctin (e.g., near Lynchburg, according to Spencer, Bowring, and Bell, 1989), but this is the only location that I’m aware of where they have been reported in northern Virginia. The implications are not all that tremendous: just that a portion of the Catoctin erupted subaqueously, but it would be a neat thing to show students, especially seeing how close the outcrop is to safe parking.

Well, I’ve been to this area a half-dozen times, and I’ve never been able to find those damn pillows. It’s frustrated me, but I had an additional impetus this time around: I ran into Jodie Hayob, the petrology professor from Mary Washington University, who was out there with her students for the day. First thing we said to one another? You guessed it: “Did you find the pillows?”

While the students ate their lunches, I went off downhill (to the west), exploring and looking for these confounded pillows. Pretty soon, I found something that looked vaguely pillowy, at least in terms of have a well-defined “crust” with a dark interior (click through that link for a fine Canadian pillow, courtesy of Ron Schott). Prepare yourself for a lot of photos today… Here’s what I saw:

A few meters further downhill, I found another outcrop of the same stuff, this one veiled in a thin layer of algae (ahh, the joys of east coast geology!):

Little double-ridges which varied in parallel, defining small chunks of rock. Could these be the fabled pillows? But they’re …so small! They’re almost pincushions! I know they say size doesn’t matter, but it’s hard for me to picture a volume of lava this small hitting water and “inflating” to such a puny volume with a nice quenched glassy rind, but then having the interior to stay hot enough to crystallize into basalt. Hmmm. Starting to think something’s fishy with this subaqueous tale…

I then found a nice big cliff, 10 meters high and 20 meters wide, which was made of almost nothing but these structures. Here’s some of them highlighted by the sun (the boundary ridges weather out in high relief), despite being obscured beneath several layers of lichen:

A relatively clean, but relatively unweathered sample:

Aha, now that’s better:

The next two show more of a “classic” Catoctin coloring: chlorite green when fresh, with buff weathered surfaces on the outside:

Zooming in on one small, skinny purported “pillow”:

I climbed back up and coerced some students into joining me to check these weird things out, and they clambered down. Danny W. found a nice chunk of float which showed one of the “pillows” in three dimensions. Check it out at the top of this sample:

Three-dimensional extension courtesy of Photoshop; red line shows the long axis of this oblate ~ellipsoid plunging towards the camera. (Lara laughs in the background…)

Okay; two more… Check out how angular the boundaries of these “pillows” are:

…Seeing that angular “break” on the left led me to realize that not only are these things too small* to be pillows, they also don’t have the right shape. Instead of being “pillowy,” (i.e., round) they are very angular, defined by edges that are aligned in a common direction and continue from one to the next.

* Where “too small” is defined as “smaller than anything Callan has seen before.”

I sketched in some of these planar edges:

To me, it looks like what’s happening here is that original homogeneous rock of the Catoctin Formation fractured, and then fluids flowed along those fractures, altering the rock that the fluids came into direct contact with. This produced the “double ridge” of buff-colored rock (on either side of the fracture), with the less-altered greenstone interiors being beyond the reach of these altering fluids. The intersection of the various joints and their subsequent boundary-defining alteration would look something like this example (from the online structure photo collection of Ben van der Pluijm): definitely click through to check it out.

In other words, I interpret these structures to be secondary, not primary. The end result is something that looks a lot like “boxwork” (again, please click through to get a sense of what I’m suggesting here): a phenomenon that occurs when limestone fractures, more resistant mineral deposits are precipitated in those fractures, and then the limestone blocks are dissolved away, leaving behind the “fractures” as planar ridges separating little “boxes” from one another.

Here’s two photos of boxwork, one whole-sample, one zoomed-in. This sample is in the USGS library in Reston, Virginia, and both photos were taken at my request by Bill Burton of the Survey. (Thanks Bill!)

At Little Stony Man, of course, the greenstone hasn’t “dissolved” away, but it does appear to be weathering more rapidly than the resistant buff-colored edges to these blocks, producing a distinctly boxwork-like effect.

Let’s look back at some of my field photos again, this time with the pillow boundaries highlighted in red…

(…I definitely could have hit a few more boundaries on that last one; forgive me for being haphazard and slapdash…)

This exercise convinced me that these things are not pillows, but some sort of fluid-rock interaction effect that took place on a complex fracture network. There’s no reason for the sharp edges of two adjacent pillows to be perfectly parallel and aligned.And it strains credulity to imagine ultra-tiny pillows in the first place (the size of my fingernail? Come on!).

I’ve e-mailed one of the authors of the original paper claiming pillows in this area with a link to my photos asking if these things are what he and his co-author were referring to, but I haven’t heard back anything. (I’ll update this post if he responds.) I might be totally off base here, but I can see how someone could make the claim that these were pillows. It’s just not a claim that convinces me, based on these outcrops.